Pathogens, allergens and odor-causing agents, including pathogenic microbes, molds, mildew, spores, and organic and inorganic pollutants, are commonly airborne or on contact surfaces in a wide range of environments. These substances can cause discomfort and, in some situations, serious illness and death to those who inhale or come in contact with them.
Microbial control and disinfection in environmental spaces is generally desirable because it improves the cleanliness and healthiness of the area. Additionally, disinfection of the air and surfaces associated with a medical clean room, an operating room, a food processing environment, certain types of pharmaceutical or bio labs and the like is a necessity. Numerous techniques, devices and procedures have been used to disinfect spaces and areas in order to purify air and disinfect surfaces initially and to keep them disinfected for extended periods of time. Many of these known techniques, devices and procedures are complex, cumbersome, expensive and relatively ineffective over the long term.
For example, it is known that Reactive Oxidizing Species (“ROS”), which are chemically reactive molecules containing oxygen such as those produced by a photocatalytic oxidation processes, are able to oxidize organic pollutants and kill microorganisms on contact. More particularly, the products of photocatalytic reactions, such as hydroxyl radicals, hydroperoxyl radicals, chlorine, hydrogen peroxide, and ozone, are known to be capable of oxidizing organic compounds and killing microorganisms. There are, however, existing limitations to the already known methods and devices available to those skilled in the art. The existing limitations are due to both their limited efficiency as well as because of potential human safety issues. “ROS” is a term often used to describe highly activated molecules created in air that result from ambient humid air being exposed to a specific bandwidth of ultraviolet (UV) light.
With respect to the use of UV light alone, light or radiation in the ultraviolet range (i.e. 10-400 nm) emits photons at a frequency that has sufficient energy to break chemical bonds when absorbed certain compounds. UV light at wavelengths between 250-255 nm is routinely used as a biocide. UV light between about 181 nm to about 187 nm is often used to break down certain molecules found in air in order to produce ozone. UV light can also be used in certain circumstances to produce ozone. Production of ozone using UV light is often a method that is competitive with an electrical corona discharge technique that is also used to produce ozone.
UV radiation and ozone are both sometimes used to disinfect community water systems. Ozone is known to be used to help disinfect and treat industrial wastewater and water cooling towers.
Hydrogen peroxide is also known to have antimicrobial properties and has been used in aqueous solution for disinfection and microbial control.
Many attempts to disinfect air within a room by using hydrogen peroxide in its gas or vapor phase have been made. However, the attempts have been hampered by technical hurdles associated with the desire to produce “purified” hydrogen peroxide. More particularly, vaporized aqueous solutions of hydrogen peroxide generally produce an aerosol of micro droplets composed of aqueous hydrogen peroxide solution.
Various processes for “drying” vaporized hydrogen peroxide solutions produced a hydrated form of hydrogen peroxide that was not very useful. The hydrated hydrogen peroxide was determined to be undesirable because the hydrated hydrogen peroxide molecules were surrounded by water molecules bonded by electrostatic attraction and/or London Forces. It was further determined that the ability of the hydrogen peroxide molecules to interact directly with the environment via an electrostatic force was greatly attenuated by the water molecules bonded to the hydrogen peroxide molecules. Accordingly, past efforts have been directed at reducing or eliminating the water molecules being bonded to the hydrogen peroxide in order for the hydrogen peroxide molecule to be able to interact with, react with and disinfect organic pollutants and microorganisms.
Additionally, an important drawback of using vaporized hydrogen peroxide is that the concentrations needed and created were generally well above the 1.0 ppm OSHA workplace safety limit. Thus, using vaporized hydrogen peroxide to disinfect organic pollutants and to kill microorganisms is unsuitable for use in work areas or environments that are occupied by workers.
Photocatalysts have been used in the past to reduce or eliminate organic pollutants in fluid. Such photocatalysts include, but are not limited to, TiO2, ZnO, SnO2, WO3, CdS, ZrO2, Sb2O4 and Fe2O3. Of these, titanium dioxide (TiO2) is chemically stable, has a suitable bandgap for UV/Visible photoactivation, and is relatively inexpensive. The photocatalytic chemistry of titanium dioxide has therefore been the object of extensive studies over the past thirty years for its ability to reduce or eliminate organic and/or inorganic compounds that are hazardous to humans from contaminated air and water.
Photocatalysts have been used to produce hydrogen peroxide gas for release into an environment, because the photocatalysts also generate hydroxyl radicals inexpensively from water when activated by UV light of sufficient energy. Prior uses of photocatalysts, however, have primarily focused on the generation of plasma containing many different reactive chemical species. Further, the majority of the chemical species in the photocatalytic plasma are reactive with hydrogen peroxide, and therefore inhibit the production of hydrogen peroxide gas due to the simultaneous reactions that also destroy or tear down hydrogen peroxide molecules. Also, since hydrogen peroxide is a very reactive molecule, any organic gases that are introduced into the plasma inhibit hydrogen peroxide production both by direct reaction with hydrogen peroxide and by reaction of their oxidized products with hydrogen peroxide.
Photocatalytic reactors themselves also limit the production of vaporized hydrogen peroxide for release into the environment. This is due to the fact that hydrogen peroxide has greater chemical potential than oxygen to be reduced as a sacrificial oxidant. Thus in photocatalytic reactors, hydrogen peroxide is preferentially reduced back to hydrogen and oxygen as it moves downstream, within a reactor, as rapidly as the hydrogen peroxide is produced by the oxidation of water.
Oxidation:2 photons+2H2O→2OH.+2H++2e−2OH.→H2O2 
Reduction:2OH.+2H++2e−→2H2O
Additionally, several side reactions generate a variety of species that become part of the photocatalytic plasma and inhibit the production of hydrogen peroxide gas that can be released into an environment.
The wavelengths of light that activate photocatalysts are energetic enough to weaken or break the peroxide bond in a hydrogen peroxide molecule and inhibit the production of vaporized hydrogen peroxide that can be released into an environment. Further, the practice of using wavelengths of light that produce ozone introduces yet another species into the photocatalytic plasma that destroys hydrogen peroxide.O3H2O2→H2O+2O2 
In practice, the use of photocatalysts has focused on the production of plasma, often containing ozone, which is then used to oxidize organic contaminants and microbes. Since these plasmas are primarily only effective within the confines of the reactor itself, these devices are designed to pass only air through the reaction chamber for disinfection. See, for example, U.S. Pat. No. 6,955,791. As such, they are of limited use in disinfecting either large spaces or the surfaces of objects.
The plasmas also have a limited chemical stability beyond the confines of the reactor where they were created and further actively degrade hydrogen peroxide gas therein.
Furthermore, since with this prior design, the plasma is only really effective within the confines of the reactor itself, many prior designs try to maximize the residence time of the “contaminated air flow” within the reactor in order to facilitate a more complete oxidation of organic contaminants and microbes as they pass through the reactor. But, since hydrogen peroxide has such a high potential to be reduced, the maximized residence time concomitantly results in a minimized hydrogen peroxide production resulting in an ineffective decontamination device.
Also, most uses of photocatalysts produce environmentally objectionable chemical species. The first objectionable chemical species among these is ozone itself, which is an intentional product of many systems. Ozone, however, is potentially harmful when inhaled by humans and ozone levels thereof are strictly regulated.
Moreover, since organic contaminants that pass through a reactor are seldom oxidized in a single exposure, multiple air exchange devices may be necessary to achieve full oxidation of organic contaminants and microbes to carbon dioxide, water and other non-organic contaminants. When incomplete oxidation occurs, various aldehydes, alcohols, carboxylic acids, ketones, and other partially oxidized organic species can be produced in the reactor. Often, prior photocatalytic reactors can actually increase the overall concentration of organic contaminants in the air by fractioning large organic molecules into multiple small organic molecules, such as formaldehyde.
Other prior attempts to use ozone and hydrogen peroxide for disinfection have sought to control the temperature and humidity of the environment being cleaned. For example, U.S. Pat. No. 7,407,624 discloses a method for abating allergens, pathogens, odors and volatile compounds in a sealed enclosure using specific concentrations of ozone and hydrogen peroxide in the sealed environment at a specific temperature and specific humidity. This method, however, is not practical for use in an area the size of a room, such as a medical surgery room because of the need to seal the environment being disinfected and because the levels of ozone are far too high for human safety.
As such, there remains a need in the art for an effective method of controlling and/or reducing the level of one or more pathogens, allergens, organic or inorganic pollutants, and/or odor-causing agents from the air and surfaces in an environment where humans live or work.